For use as washers for bolts and nuts, a great variety of washers including standard plain, spring, wave, teethed and conical washers have been proposed and practically employed. However, these prior art washers have been found unsatisfactory with respect to practicability and cost.
The reason is that in the production of the conventional washers for such purposes, due and sufficient consideration has not been given to the absolutely necessary clamping force and proper bearing stress, which are essential for fastening means, and the technological basis on which a tightening torque required for satisfying such clamping force and bearing stress is determined, and also the simplification of the control of tightening.
Conventional bolts and nuts produced by proper facilities and with proper design would perform their expected functions as fastening means if they are tightened the proper control of tightening. However, the conventional tightening method of controlling the tightening of bolts and nuts complicated substantially because the torque to be designed for a bolt or a nut varies within a substantially wide range and very often depends upon the material of a heat-treatment performed on a member to be tightened, conditions on the contact faces of the bolt or nut and the member to be tightened and the co-rotation of the bolt or nut and a washer, if the latter is used.
Bolts and nuts as tightening means are required to be kept tight under all conditions, with no inadvertent loosening.
Although not usually considered as a serious matter, it is important that the tightening means, such as a bolt or a nut, under vibratory conditions is not under permanent strain at the area of its face contacting the member to be tightened.
In order to prevent the occurrence of this permanent strain, the area of the bearing surface of the bolt head or nut must be properly designed and provided according to variations of the member to be tightened.
For the above reason, it is necessary that a washer maintain a constant frictional resistance between the bearing surface of a bolt head or a nut and the upper contacting surface of the washer and between the lower contacting surface of the washer and the contact surface of the member to be tightened, respectively, regardless of the material of a member to be tightened. The washer must also have an area and a thickness to ensure a proper bearing stress with no slip between the lower contacting surface of the washer and the contact surface of the member to be tightened, respectively corresponding to variations of the necessary clamping force. If possible, the washer is further required to work for locking or provide a great frictional resistance in the loosening direction of the bolt or nut.